Hybrid optical mirror

ABSTRACT

A hybrid mirror for reflecting high power optical beams comprises a reaction sintered silicon carbide substrate to which is bonded a plurality of silicon faceplates containing a plurality of coolant passages.

TECHNICAL FIELD

[0001] The field of the invention is that of cooled optical mirrors.

BACKGROUND ART

[0002] In the field of cooled mirrors for high-power lasers and opticalsystems, the state of the art has changed from copper mirrors tomolybdenum and from single-pass cooling to multiple layers of coolantchannels.

[0003] Prior art mirrors have been fabricated with faceplate, heatexchangers and substrate of the same material for the obvious reasonthat differential expansion increases the distortion of the mirrors. Thechange from copper to molybdenum produced a better compromise between ahigh coefficient of thermal conductivity and a low coefficient ofthermal expansion, at the cost of tackling the problems of working withmolybdenum.

[0004] Work with different forms of heat exchangers has shown that stateof the art molybdenum mirrors offer rapidly diminishing returns in thatfaceplate distortion decreases only slowly as cooling capacityincreases. It is evident to those skilled in the art that futurerequirements for high flux, highly cooled mirrors that have extremelylow distortion cannot be met by further refinements of presentmolybdenum mirrors. For the particular case of a laser operating inspace, where the distortion requirements are more severe than fortypical earth-based applications and where there is an added weightrequirement, molybdenum mirrors are even less capable of meeting thesystem requirements.

[0005] Those skilled in the art have long sought an improved materialfor high-power mirrors, one approach being that disclosed in U.S. Pat.Nos. 4,142,006 and 4,214,818 issued to W. J. Choyke and R. A. Hoffman,showing the use of a hot-pressed silicon carbide mirror, in which theoptical surface is either polished onto the substrate or is polishedonto a vapor deposited layer of silicon carbide. These mirrors have theadvantage that using the same material for the substrate and thefaceplate reduced optical distortions caused by dissimilar thermalexpansion. They also have the advantage that silicon carbide has lessthan one-third the density of molybdenum. These mirrors suffer adisadvantage, however, in that the thermal conductivity of siliconcarbide is less than molybdenum.

DISCLOSURE OF INVENTION

[0006] The invention relates to a lightweight mirror for reflecting highpower optical beams comprising a reaction sintered silicon carbidesubstrate supporting a faceplate formed by one or more plates of siliconcontaining coolant passages therein.

BRIEF DESCRIPTION OF DRAWING

[0007]FIG. 1 illustrates an embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0008] When a high power optical beam is reflected off a mirror, thenonuniform energy flux in the beam generates heat at the mirror surfacewhich is of different magnitude in different regions of the mirror. Themirror absorbs the heat and then expands at a different rate indifferent areas, resulting in a distortion of the mirror shape. It haslong been a key problem in the art of designing laser mirrors to reducethe distortion produced under beam load.

[0009] The standard combination of materials in the prior art has been amolybdenum faceplate containing passages for flowing coolant material,bonded to a molybdenum substrate, molybdenum having the best combinationof thermal conductivity to conduct heat away from the reflective surfacecombined with sufficient rigidity of the substrate to resist thermalbending under beam loading.

[0010]FIG. 1 illustrates an embodiment of the invention in whichsubstrate 110 is formed from reaction sintered silicon carbidecontaining 5-25% free silicon. This substrate is bonded to a faceplatepackage consisting of a manifold plate 130, and faceplates 120 and 122,the uppermost surface of faceplate 122 being polished to an opticalfinish. Faceplates 120 and 122 are plates of crystalline silicon whichare sliced from an ingot, lap ground to appropriate flatness, andpolished to a damage free surface condition. The silicon plates 120 and122 have a number of coolant passages 132 machined therein by chemicaletching, ultrasonic machining or a combination thereof. The coolingpassages will typically be semicircular or rectangular in shape with adepth of 0.030 in. in a plate which is 0.045 in. thick. The landsbetween adjacent cooling passages are typically 0.015 in. wide. Plate130 is a manifold plate composed of crystalline silicon prepared asdescribed previously with entry ports machined so as to introducecoolant to the faceplate. Manifold plate 130 is typically 0.015 in.thick.

[0011] Silicon faceplates 120 and 122, and silicon manifold plate 130are bonded together metallurgically by placing gold foil at the plateinterfaces and heating the assembly in vacuum at a temperature between400° C. and 1200° C. When using suitable bonding parameters, the goldwill alloy with silicon and diffuse away from the interface to theextent that a regrowth layer of silicon will crystallize epitaxially atthe interface resulting in a bond joint of strength equivalent to thesilicon substrates. Typical conditions for successful bonding are 60min. at 1100° C. An identical bonding process may also be utilized tobond the remaining face of the silicon manifold plate 130 to a reactionsintered silicon carbide substrate 110, since the substrate containsfree silicon which will likewise alloy with gold and result incrystallization of a regrowth layer of silicon at the interface.

[0012] When SiC is reaction sintered, a “skeleton” of SiC is formed withfree silicon interspersed through it. The bonding agent, gold oraluminum, penetrates through the skeleton to alloy with the freesilicon. A material such as SiC hot-pressed to 99% of theoreticaldensity would not be a suitable material because it would not have thefree silicon available.

[0013] The use of gold is not essential, and other materials, such asaluminum, may be used.

[0014] State-of-the-art mirrors are produced from molybdenum substratesand faceplates, since molybdenum has the best combination of lowcoefficient of thermal expansion, high thermal conductivity and highelastic modulus, together with being less difficult to fabricate thanalternative materials.

[0015] Those skilled in the art would not ordinarily think to combinedifferent materials for the faceplate and substrate because it is wellknown that dissimilar coefficients of expansion will aggravatedistortion and bonding problems. It is surprising, therefore, thatdetailed calculations of thermal loading effects have predicted asignificant improvement in distortion for this hybrid mirror comparedwith molybdenum, for the same volume of coolant in a steady-statesituation.

[0016] Silicon has an elastic modulus in the range of 19-27 Msi, with(111) oriented single crystal silicon having an elastic modulus of 24Msi, compared with a value of 47 Msi for molybdenum. Reaction sinteredSiC typically has an elastic modulus in the range of 45-55 Msi. Thethermal bending moment of the entire mirror structure is reduced byhaving a low modulus faceplate on a higher modulus substrate or backupstructure. Thus, though both Si and reaction-sintered SiC have elasticmoduli no better than molybdenum, the combination described above offerssignificantly improved optical performance.

1. A cooled mirror comprising: a ceramic substrate having a top surface;a faceplate bonded to said substrate, said faceplate comprising at leastone silicon plate having a plurality of coolant passages therein, thatsurface of said faceplate farthest from said top surface of saidsubstrate being polished to an optical finish.
 2. A mirror according toclaim 1, in which said substrate is formed from siliconized siliconcarbide containing between 5 and 25 per cent free silicon and that oneof said at least one plate of silicon closest to said substrate isbonded to said substrate by a metallurgical bond.
 3. A mirror accordingto either of claims 1 or 2, in which said faceplate and substrate arebonded metallurgically with a bonding metal at a temperature in excessof the bonding metal-silicon eutectic temperature.
 4. A mirror accordingto claim 3, in which said bonding metal is gold.
 5. A mirror accordingto claim 3, in which said bonding metal is aluminum.